# std::generate

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class ForwardIt, class Generator > void generate( ForwardIt first, ForwardIt last, Generator g ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class Generator > void generate( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Generator g ); | (2) | (since C++17)

1) Assigns each element in range `[`first`,` last`)` a value generated by the given function object g.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range of elements to generate
policy | - | the execution policy to use
g | - | generator function object that will be called. The signature of the function should be equivalent to the following:
Ret fun(); |  |
The type Ret must be such that an object of type ForwardIt can be dereferenced and assigned a value of type Ret. ​
Type requirements
- `ForwardIt` must meet the requirements of LegacyForwardIterator.

### Complexity

Exactly std::distance(first, last) invocations of g() and assignments.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

    template<class ForwardIt, class Generator>
    constexpr //< since C++20
    void generate(ForwardIt first, ForwardIt last, Generator g)
    {
        for (; first != last; ++first)
            *first = g();
    }

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <vector>

    void println(std::string_view fmt, const auto& v)
    {
        for (std::cout << fmt; const auto& e : v)
            std::cout << e << ' ';
        std::cout << '\n';
    };

    int f()
    {
        static int i;
        return ++i;
    }

    int main()
    {
        std::vector<int> v(5);

        std::generate(v.begin(), v.end(), f);
        println("v: ", v);

        // Initialize with default values 0,1,2,3,4 from a lambda function
        // Equivalent to std::iota(v.begin(), v.end(), 0);
        std::generate(v.begin(), v.end(), [n = 0] () mutable { return n++; });
        println("v: ", v);
    }

Output:

    v: 1 2 3 4 5
    v: 0 1 2 3 4

### See also

fill | copy-assigns the given value to every element in a range (function template)
generate_n | assigns the results of successive function calls to N elements in a range (function template)
iota (C++11) | fills a range with successive increments of the starting value (function template)
ranges::generate (C++20) | saves the result of a function in a range (algorithm function object)

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